How to include a script from an external source?

nironan's suggestion at #3 has worked really well for me, although worth noting that the JS has to be executed in the footer for the document.body.appendChild(scriptTag); to work.

I had to add Plausible to a D7 site (which only has a D8+ module), couldn't add the recommended snippet to the theme, so used the following JS executed inline in the footer (following provided to those landing here via a search listing):

var scriptTag = document.createElement('SCRIPT');
scriptTag.src = 'https://plausible.io/js/script.js';
scriptTag.dataset.domain = "MY DOMAIN";
scriptTag.async = true;
scriptTag.defer = true;
document.body.appendChild(scriptTag);